Research on carbon nanotubes (CNTs) is vigorously being conducted ever since light emission by the carbon nanotubes was observed by photoluminescence measurement involving light excitation to observe light emission. In recent years, light emission by the carbon nanotubes due to current injection was observed, and as a consequence, carbon nanotubes have also come to be expected as light emitting devices. Light emitting devices using carbon nanotubes have been reported in Non-Patent Literatures 1 to 8. However, research and development on high-speed modulation performance of the carbon nanotube light emitting devices had not been reported at all and the field thereof had not yet been explored.
A light emission mechanism of the carbon nanotube light emitting devices is divided into two categories: (a) light emission by electron-hole recombination; and (b) light emission by blackbody radiation.
(a) Light Emission by Electron-Hole Recombination
Today, vigorous research is mainly conducted with respect to the light emission mechanism by electron-hole recombination. The light emission by electron-hole recombination is implemented in such a manner that an electron and a hole are excited in a semiconductor carbon nanotube by a certain method and the excited electron and hole are recombined to thereby emit light. This is the light emission principle that applies to light emitting diodes (LEDs) in solid-state semiconductors today.
Reported as the excitation method are (i) electron/hole injection excitation, (ii) impact excitation, and (iii) thermal excitation. Method (i) is to inject an electron and a hole from opposite two electrodes formed on a carbon nanotube and to emit light by recombination of the injected electron and hole. Method (ii) is to inject a hole (or an electron) having high kinetic energy from an electrode, use energy, which is produced when the kinetic energy is lost, to generate an electron-hole pair (exciton), and to emit light by relaxation of the exciton. Method (iii) is to excite an electron by thermal energy created by heating, such as Joule heat when current passes through a carbon nanotube, and to emit light by relaxation of the electron with a hole.
(b) Light Emission by Blackbody Radiation
All substances radiate electromagnetic waves with heat at temperatures over an absolute zero-point (blackbody radiation). An emission spectrum in the blackbody radiation is described based on Planck's law, and energy of the thermal radiation conforms to Stefan-Boltzmann law stating that the energy of thermal radiation is proportional to the fourth power of temperature T. Today, the blackbody radiation is used in, for example, tungsten filaments for electric bulbs, which are for use in lighting and the like.
Carbon nanotubes have also been reported to be bundled into the form of filaments so that heating the carbon nanotubes by passing of current makes them emit light like filaments (Non-Patent Literature 9). However, in the case of light emission in the form of filaments, on-off modulation cannot be performed at high speed just as in the case of tungsten electric bulbs. There are also reports that blackbody radiation was observed by passing current to multi-walled carbon nanotubes in the form of a thick bundle of about several μm (Non-Patent Literature 10) and to a thick multi-walled carbon nanotube of about 13 nm in diameter which is disposed on a substrate (Non-Patent Literature 11). These reports indicate that the blackbody radiation conforms to Planck's law. However, high speed modulation is difficult in thick carbon nanotubes, and therefore high speed modulation is not yet attempted therein.
Unlike in the case of light emitting diodes and laser diodes, blackbody radiation has a characteristic that a continuum spectrum (continuum spectrum light source) in a wide wavelength range can be obtained. However, in the blackbody radiation by conventional filaments and the like, high-speed modulation performance is not attained. In the present circumstances, therefore, there is no continuum spectrum light source that can be modulated at high speed.
The light emitting devices reported so far are roughly categorized into two types: light emitting devices using one carbon nanotube; and light emitting devices formed with a carbon nanotube thin film (carbon nanotube network) including a large number of carbon nanotubes.
The light emitting devices using one carbon nanotube have been reported in Non-Patent Literatures 1 to 6, while in Non-Patent Literatures 7 and 8, light emission is observed by using a bundle of carbon nanotubes which are referred to as bundled carbon nanotubes formed by tangling several carbon nanotubes into one bundle. The light emitting device using one carbon nanotube or one bundle of carbon nanotubes has characteristics that single light emitted thereby is easily taken out and that its spectrum width is narrow. However, it is necessary (1) to observe a substrate with the carbon nanotube grown thereon under a microscope to find out the one carbon nanotube, or (2) to select, out of a large number of devices which were fabricated, one device accidentally formed as a single electrode. Accordingly, yields of devices are generally poor.
Contrary to the above, the light emitting device using a carbon nanotube thin film (or network), which has been proposed by the applicant in Patent Literature 1, can be fabricated with sufficient yields. However, the light emitting device formed with a carbon nanotube thin film has a characteristic that an emission spectrum thereof becomes broader than the light emitting device formed with one carbon nanotube. In Non-Patent Literatures 7 and 8, light emission from thin-film carbon nanotube devices was also reported.